The invention relates to new and useful improvements in X-ray diagnosis machines. More particularly, the invention relates to an X-ray diagnosis machine having an X-ray source and an image recording device with a photo-timer. The photo-timer has at least one measurement field in a radiation detector plane which is essentially parallel to the image plane.
Modern X-ray recording machines are increasingly making use of a photo-timer to control exposure (intensity and duration). An important component of such photo-timers are so-called measurement fields (sensors). These measure the radiation impinging on the X-ray detector located behind the trans-irradiated object and control the beam generation via an electronic system. The purpose thereof is ensuring correct exposure of the X-ray detector, which in conventional systems is often an X-ray film, but which is now often some other type of detector, for example a so-called X-ray image intensifier or other type of electronic detector (e.g. aSi/Se).
The measurement fields or sensors are typically arranged at fixed positions distributed over a surface (in this respect reference is made, for example, to the product descriptions of Siemens: DIGISCAN 2T or Philips: THORAVISON). The objects to be radiographed must therefore be brought into the beam path between the X-ray source and the detector so as to cover at least one sector of the measurement fields. This sector is then used exclusively to control the recording operation. However, such positioning squarely in front of the measurement field is not always possible when the patient is a child, or a person who is badly injured or severely disabled.
To solve this problem, the user has to take so-called "free", i.e. manual, non-automated exposures without the assistance of measurement fields. This can result in faulty exposures and an increased radiation burden for the patient, owing to the possibility that repeated pictures may need to be taken.
German Laid-Open Publication DE 33 10 971 A1 discloses an X-ray diagnosis machine which addresses this problem by arranging a plurality of measurement fields so that they are distributed over a surface corresponding to the total surface of the image recording part. A computer compares the selected radiation field size on the detector of the photo-timer with the selected measurement fields and switches off any measurement fields that are not situated completely within the radiation field. Although this arrangement circumvents the problem described above, it is very expensive in terms of both the outlay for construction and the cost and complexity of the associated electronic evaluation system.